These were all born about the time of Hales' zenith, nor did he live to see the great results they accomplished. But it should not be forgotten that Hales' chemical work made more easy the triumphant road they trod.
I have spoken of Hales in relation to chemists and physicists because, though essentially a physiologist, he seems to me to have been a chemist and physicist who turned his knowledge to the study of life, rather than a physiologist who had some chemical knowledge.
Whewell points out in his History of the Inductive Sciences that the Physiologist asks questions of Nature in a sense differing from that of the Physicist. The Why? of the Physicist meant Through what causes? that of the Physiologist—To what end? This distinction no longer holds good, and if it is to be applied to Hales it is a test which shows him to be a physicist. For, as Sachs shows, though Hales was necessarily a teleologist in the theological sense, he always asked for purely mechanical explanations. He was the most unvitalistic of physiologists, and I think his explanations suffered from this cause. For instance, he seems to have held that to compare the effect of heat on a growing root to the action of the same cause on a thermometer was a quite satisfactory proceeding. And there are many other passages in Vegetable Staticks where one feels that his speculations are too heavy for his knowledge.
Something must be said of Hales' relation to his predecessors and successors in Botanical work. The most striking of his immediate predecessors were Malpighi 1628—1694, Grew 1628—1711, Ray 1627—1705, and Mariotte (birth unknown, died 1684); and of these the three first were born one hundred years before the publication of Vegetable Staticks. Malpighi and Grew were essentially plant-anatomists, though both dealt in physiological speculations. Their works were known to Hales, but they do not seem to have influenced him.
- Black's discovery of CO2, however, was published in 1754, seven years before Hales died, but Priestley's, Cavendish's and Lavoisier's work on O and H was later.
- 1837, III. p. 389.
- Vegetable Staticks, p. 346.